The photolithographic production of semiconductors utilizes a series of photomasks to produce the features on the semiconducting wafers. In each photolithographic step, a photomask is imaged onto the prepared semiconducting wafer. Once a photomask is imprinted onto the wafer, additional processing may be used to modify the semiconductor wafer material in the pattern of the photomask. In subsequent steps, additional photomasks may be imaged onto the wafer. High-precision alignment of the subsequent photomasks relative to features produced using the previous photomasks promotes the quality of structures manufactured on the semiconductor wafer. Metrics to evaluate the quality of structures manufactured on a semiconductor wafer may include dimensional precision of a single feature, dimensional precision of the location of a one feature relative to other features, or combinations thereof. Some dimensions of the features on the photomask may be referred to as critical dimensions (CDs), and the placement of features on the photomask relative to the other features may be referred to as overlay. For the most critical photomasks, the overlay requirement can be a few nanometers or less across the entire length of the mask.
The properties of photomasks, such as dimensional properties and material properties, for example, may vary within ranges resulting from tolerances applied during the manufacturing process. For example, the optical performance characteristics of partially absorbing films may depend on the material composition of the film and a thickness of the film. Further, the CDs and overlay of the mask may vary depending on the precision of the write tools, the etching processes, the number and type of cleaning processes, and the temperatures and stresses applied on the photomask during fabrication. Variability in the characteristics of the unprocessed photomasks (e.g., compositional and film stress differences), can also contribute to the final variances in CD and overlay by varying the response of the photomask to the manufacturing processes.
The mask-to-mask differences ultimately lead to variations in the product created using the photomasks. For this reason, limits or ranges of performance criteria are applied to distinguish defective photomasks from photomasks that are acceptable for production use. If a photomask performance is outside an acceptable limit the photomask is rejected and another mask is built.
In addition to fabrication variances, the performance characteristics of photomask can change due to use in production. During production use, the photomask may be exposed to intense radiation, including ultraviolet (UV) radiation, deep ultraviolet (DUV) radiation, or extreme ultraviolet (EUV) radiation. The radiation can cause photo-degradation of the photomask, particularly of the thin film. Also, photomasks may require cleaning after use in production for a period of time. These cleaning processes may alter the characteristics of the photomask, and there may be a limit to the number of times a mask can be re-cleaned and still meet specifications.
A process for reducing the mask-to-mask variability of one or more of the critical process characteristics of a photomask could potentially improve the overall production yield of a semiconductor fabrication process using photomasks. Firstly, correcting out-of-specification masks and making them viable for production use could help to avoid production delays and costs associated with fabrication of new photomasks to replace the out-of-specification photomasks. Secondly, reducing the variation in one or more performance characteristics may result in tolerance margin that can be used to expand the acceptable range of another characteristic. Thus, reducing the variability of any critical photomask characteristic may provide the benefit of reducing the variability in the printed features on the product wafers, which improves device yield and performance.
As an example, it would be advantageous to improve the overlay of a photomask to improve the precision of feature placement on the wafer, which could be used to correct out of specification masks, thereby improving mask yield. In addition, it would be advantageous to reduce the mask-to-mask variation in overlay for sets of masks that are printed on the same wafer. By improving the alignment of the features from multiple masks on the wafer, an otherwise defective increase in the variability of another critical parameter may become acceptable.
As the photomask industry moves toward multiple patterning, there is an increased requirement for overlay precision between critical mask layers. In multiple patterning, two or more masks are used in combination to create reduced feature sizes on the printed wafer. The reduced feature sizes may require both reduced errors in individual masks and reduced overlay errors between sets of masks. Therefore, it would be particularly advantageous to develop a process to improve overlay for photomasks used in multiple patterning.
It will be appreciated that this background description has been created by the inventors to aid the reader, and is not to be taken as an indication that any of the indicated problems were themselves known in the art.